The present invention relates to a high-strength inorganic fiber having excellent photocatalyst function and a process for the production thereof. Specifically, it relates to an inorganic fiber which shows excellent photocatalyst activity by visible light irradiation and a process for the production thereof.
There have been made many attempts to decompose and purify various environmental pollutants by using an photocatalyst effect of a semiconductor typified by titanium dioxide. When the above photocatalyst effect is utilized, conventionally, titania crystal grains are fixed on a substrate. However, many problems occur on a bonding method so that in recent years a keen attention is focused on a titania fiber free from the fixation problem.
For example, JP-A-5-184923 discloses a method of synthesizing a fiber composed of crystals of anatase type titania and vanadium oxide, which method comprises dissolving titanium alkoxide and a vanadium compound in alcohol, carrying out hydrolysis to prepare a sol-like material, forming the sol-like material into a fiber-like material, gelating the fiber-like material and heat-treating the gel in the range of from 200 to 700xc2x0 C. Examples of the above JP-A-5-184923 mainly discribe a fiber containing titania and vanadia and further containing quantities of a silica component. With regard to catalyst activity as a fabric using the above fiber, JP-A-5-184923 shows only the catalyst activity of a fabric obtained by mixing only 20% of the above fiber into an E glass made of silica.
Conventionally, it is known that a titania fiber synthesized by the sol-gel method is extremely fragile. As a study for increasing the strength thereof, for example, xe2x80x9cYogyo-Kyokai-shixe2x80x9d, vol 94 (12), pages 1,243 to 1,245, (1986) describes coexistence of a silica component. The above method described in the examples of JP-A-5-184923 exactly adopts this method. Further, JP-A-11-5036 publication discloses a silica-titania fiber for a photocatalyst according to the sol-gel method and a production process thereof. In this case, the fiber also has an extremely low strength of 0.1 to 1.0 GPa.
In addition to the above methods, the following reports have been disclosed as a production process of titania. For example, xe2x80x9cJournal of Material Science Lettersxe2x80x9d 5 (1986), 402-404, reports a method of synthesizing a gel-like titania fiber (anatase) in which hydrochloric acid coexists in an alcohol solution of titanium alkoxide, hydrolysis is carried out to obtain a colloidal substance, the colloidal substance is spun, and the spun fiber is heated under a humidified atmosphere and then temperature-increased in air to obtain a gel-like titania fiber.
Further, xe2x80x9cThe American Ceramic Society Bulletinxe2x80x9d, May 1998, 61-65, reports a method of producing a titania fiber by adding water to fine particles of titania to obtain a slurry, mixing the slurry with viscose to prepare a viscous fluid, forming the viscous fluid into a fiber and calcining the fiber in air under heat at a high temperature.
Each of these fibers is formed through an agglomeration step of primary particles of titania so that the inside of each fiber has a serious defect. Even when photocatalyst function is recognized, it is extremely fragile. Accordingly, it is required to solve many problems for practical uses. Further, in systems where a silica component coexists in order to improve strength, titania and silica exist in a mixed state so that these systems can not provide sufficient photocatalyst activity when compared with titania alone. This is also a significant problem against practical uses.
When a photocatalyst fiber is used as a filter, it is naturally preferred that the photocatalyst fiber has a higher fiber strength since the photocatalyst fiber is exposed to a high-speed gas flow for a long period of time. Particularly, in consideration of its application to a gas emitted from an aircraft engine or a motor vehicle engine, it is strongly desired to develop a fiber having high-strength photocatalyst function or thermal-catalyst function that goes beyond conventional common sense.
On the other hand, irradiation of ultraviolet light of 400 nm or less is indispensable for making titania exert photocatalyst function. In the spectral distribution of solar rays, which can be obtained on the surface of the earth, the ultraviolet region (400 nm or less) is approximately 5%, the visible region (400 to 750 nm) is approximately 43%, and the infrared region (750 nm or more) is approximately 52%. Therefore, a photocatalyst which exerts photocatalyst function in the visible region is desired for utilizing solar rays efficiently.
As a method for the above, for example, JP-A-9-192496 discloses a method in which a metal element such as V, Cr, Mn, Fe, Co, Ni, Cu, etc., is doped into titanium oxide. This method produces a photocatalyst by adding the above dopant or its precursor to titanium oxide or its precursor such as hydroxide, chloride or nitrate and carrying out drying and then calcination. However, since it is difficult to introduce the metal as a dopant into titanium oxide homogeneously and highly dispersively, there is a problem that sufficient visible light activity can not be obtained.
Further, JP-A-9-262482 discloses a method in which titanium oxide is irradiated with a metal ion, such as Cr, V, Cu or Fe, accelerated to high energy, to introduce the metal ion into the titanium oxide. According to this method, the metal ion can be homogeneously and highly dispersively introduced into the titanium oxide. However, it requires a large-scale manufacturing equipment so that a manufacturing cost becomes high. Therefore, the problem is that the above method is unsuitable to mass production.
It is an object of the present invention to provide a high-strength inorganic fiber which exerts excellent photocatalyst activity by visible light irradiation and a process for the production thereof.
According to the present invention, there is provided a silica-based photocatalyst fiber having visible-light activity, which fiber comprises a composite oxide phase comprising an oxide phase (first phase) mainly made of a silica component and a titania phase (second phase), wherein the second phase contains a metal element other than titanium and the existent ratio of the second phase slopingly increases towards the surface of the fiber.
According to the present invention, further, there is provided a process for the production of a silica-based photocatalyst fiber, which process comprises
melt-spinning a modified polycarbosilane obtainable by modifying a polycarbosilane having a main chain structure represented by the formula, 
xe2x80x83(in which R is a hydrogen atom, a lower alkyl group or a phenyl group) and a number average molecular weight of 200 to 10,000, with an organic metal compound or melt-spinning a mixture of the modified polycarbosilane and an organic metal compound, to obtain a spun fiber;
infusibilizing the spun fiber; and
calcining the infusible fiber in air or in oxygen.